/Engine/Source/Runtime/Core/Public/Internationalization/TextFormatter.h
Incredibly low-level version ofFFormatArgumentValue::ToFormattedString. You should only be calling this if you're implementing a custom argument modifier type that itself needs to convert the argument to a string
static void ArgumentValueToFormattedString ( const [FFormatArgumentValue](API\Runtime\Core\Internationalization\FFormatArgumentValue) & InValue, const FPrivateTextFormatArguments & InFormatArgs, [FString](API\Runtime\Core\Containers\FString) & OutResult )